CHEMISTRY PAPER 1 Marking Scheme - 2017 KITUI MOCK EXAMINATION

Marking Scheme

1.   
   1.  
      1. K
      2. J 
   2. In methylbenzene HCl remains as s molecule / does not ionize hence is neutral
   3. L is a better conductor than J because L is a strong acid which ionizes fully to give more hydrogen ions
2.  
   1. Vulcanization
   2. Saponification
3.  
   1. Nitrogen 
      Argon 
   2. Used in hospitals by patients with breathing difficulties
      Used by mountain climbers and deep sea divers
      Mixed with hydrogen to provide fuel for space rockets
      Oxyacetylene flame is used in welding and cutting metals (any 2 mks)

4.  
   1. Magnesium burns with a bright light
      A white powder is formed 
   2. Mg_(s) + H₂O_(g) → MgO_(s) + H_2(g) 
   3. Remove the delivery tube form the water trough before heating is stopped to prevent sucking back
5.  
   1. Equilibrium shifts to the right ½ mk where there are few molecules ½ mk
   2. It will have no effect ½ mk because the number of molecules are equal on both sides ½ mk
6. Heat the mixture, ammonium chloride sublimes 
   Add water to the remaining mixture, NaCl dissolves leaving silver chloride 
   Filter to obtain silver chloride as the residual and NaCl as the filtrate 
   Heat the filtrate to dryness to obtain solid NaCl 

7.  
   1. Ag⁺_(aq) + e^- → Ag_(s)
   2. Q = It
      = 0.03 x 2 x 60 x 60
      = 216C 
      96500C →108g
      216C → ?
      ^{216 x 108}/₉₆₅₀₀
      = 0.242g
8.  
   

9.  
   1. Curve I- the concentration of F increases with time since F is the product
   2. After time (t) the concentration of E and F remains constant because equilibrium has been established
10.  
    1. The chloric (I) acid decomposes to form atomic oxygen ü1 mk the atomic oxygen attacks and bleaches the blue flower 1 mk
    2. 2HOCl_(aq) → O_2(g) + 2HCl_(aq) 

11. Luminous                             non-luminous
    Sooty                                    non-sooty
    Not very hot                           very hot
    Not steady                             steady (any 2)

12. KOH_(aq) + HNO_3(aq) → KNO_3(aq) + H₂O_(l) 
    

13. MgO has a giant ionic structure while SiO₄ has a grant covalent structure hence in molten state MgO conducts since it has mobile ions while SiO₄ does not
14.  
    1. X – 143 
       Y – 50
    2. Energy generations
       Food preparation
       Pest control
       Carbon dating (Any 2 each ½ mk)

15.  
    1. Brown fumes are observed  ½ mk
       Black solid is deposited ½ mk
       Blue colour fades ½ mk
    2. 2NO_2(g) + H₂O _(l) → HNO_3(aq)  + HNO _{2 (aq)}
    3. It relights a glowing splint
16.  
    1. Soapless detergent
    2.  
       1. Does not form scum
       2. Causes pollution since it is non-biodegradable
17.  
    1. The volume of a fixed mass of a gas is inversely proportional to its pressure at constant temperature 1mk. This is because particles of a gas are widely apart hence can be compressed 1mk
    2.  
       1. Low pressure ½ mk
       2. High temperature ½ mk Specify

18. In diamond each carbon atom is covalently bonded to four other carbon atoms in a rigid giant atomic structure 1mk. In graphite each carbon atom is covalently bonded to their other carbon atoms in layers ½mk. The layers are held together by weak Vander walls forces which are easily broken ½mk
19.  
    1.  
       1. An element is a substance which cannot be split into any simpler substance by any chemical process
       2. Atomic number is the number of protons in an atom 
    2. T₂(SO₄)₃
20.  
    1. Experiment 1 ½ mk
       Experiment 3 ½ mk
    2. In experiment 1, solid potassium carbonate has no free ions to conduct electricity 1mk
        In experiment 3, sugar consists of molecules which do not conduct electricity 1mk

21.  
    1. Cotton / wool / silk 1mk
    2. Are strong than natural fibres
       Not easily affected by chemicals
       Last longer / are durable
       Are cheaper (Any 1)

22.  
    1. X ½ mk - it has a completely filled outermost energy level ½ mk
    2.  
       1. W and Y  1mk
       2. YW 1mk
23.  
    1. Copper pyrites ½ mk
       Malachite ½ mk
    2.  In order to concentrate the ore
    3. Brass 1mk used in soldering wires and ornaments 1mk
24.  
    1. G
    2. The ionic radius of I is bigger than of E ü1mk since I has more energy level than E 1mk
25.  
    1. This is the energy change that takes place when one mole of a compound is formed from its constitute elements in their standard state
    2. ΔH₄ = ΔH₁ + ΔH₂ + ΔH₃
       = (–286 x 3) + (–394 x 2) – (–277)
       = – 1369 KJ/mol

26.  
    1. Oxidation state is the apparent charge that atoms have in molecules/ions
    2. (2 x 3) + 2N = 0
       2N = – 6
       N = – 3
    3. The heat is required to break the strong triple covalent bonds between nitrogen atoms
27.  
    

28. The product form the nettle plant is acidic ü1mk hence aqueous ammonia solution being basic neutralizes the acidic product. 1mk
29. Ionization energy is the energy required to remove an electron form an atom in gaseous state 1mk while electron affinity is the energy released when an in gaseous state gains an electron. 
30. Burning Magnesium produce heat energy enough to break the sulphur / oxygen bond setting oxygen free. Magnesium uses the free oxygen to continue burning.